Method of treating copper and product thereof



Patented Aug. 24, 1943 METHOD OF TREATING COPPER AND PRODUCT THEREOF Albert A. Smith, Jr., Metuchen, and John ,S. Sinai-t, Jr., Scotch Plains, N. J., assignors to American smelting and Refining- Company, New York, N. Y., a corporation of New Jersey .No Drawing. Application September 6, 1941, Serial No. 409,796

This invention relates to the production of high conductivity copper having elevated softening temperatures.

Briefly, the invention is based upon the discovery that the softening temperature of commercial tough-pitch copper can be markedly raised without appreciably lowering its conduc-- tivity by incorporating small amounts of cadmium therein and retaining it in solid solution in the copper.

It is known that additions of substantial amounts of cadmium to copper increase the tensile strength and softening temperature of copper but this is accompanied with a decrease in electrical conductivity which is undesirable. Little is known of the effects and behavior of small amounts of cadmium in copper, other than that.

such additions are ineffective for raising the tensile strength, i

We have discovered, after considerable research and experimentation, that decidedly beneficial results are obtained by incorporating relatively small amounts of cadmium in copper and causing it to be retained in solid solution therein. Commercially pure coppers containing small amounts of cadmium, treated to maintain the cadmium in solid solution as hereinafter disclosed, have softening temperatures well over 100 C. above those representative of the original copper while the decrease in electrical conductivity is practically negligible.

It is the principal object of this invention to provide a commercial procedure for producing high conductivity tough-pitch copper, useful for processing into wires, sheets and the like, having softening or recrystallization temperatures equal or superior to those of the usually employed silver bearing coppers.

As is recognized in the art, the softening which occurs during. an annealing operation depends upon the phenomenon known as recrystallization.

For any given degree of cold reduction and annealing time, the process of recrystallization proceeds over a temperature range, and the use of the term "recrystallization temperature is therefore ambiguous. In the present instance, the term softening temperature is preferred, and is defined as the temperature at which the halfhard stage is reached under the following standard conditions. Annealed wires of the material to be tested are subjected to a reduction of area of 75% by cold drawing, and the tensile strength of selected portions determined for the hard drawn condition. The fully annealed tensile strength is obtained from other sample portions annealed for one hour at 500 C. The half-hard tensile strength is then ascertained by a simple calculation. Additional samples are annealed at suitable temperatures in an oil bath equipped with automatic temperature control to furnish the data necessary to plot the portion of the tensile strength-annealing temperature curve passing-through the half-hard stage, and the softening temperature is then determined graphically.

In the preparation of copper-cadmium alloys, a master cadmium-copper alloy was made from the pure metals in the conventional manner and precalculated amounts of this alloy were added 1 to the copper to produce the final copper-cadmium product. Oxygen-free alloy material was formed by solidifying the alloys in a tightly covered graphite crucible. Oxygen-bearing alloys of similar composition were prepared and the cad- 'mium contents of all alloys were determined by chemical analysis.

Comparative test results illustrating the effect of cadmium on the softening temperature of different coppers are shown in the following tables:

TABLE I Pure Containcopper g Thermal treatment g percent Cd percent 0,

Nil Nil .162 rod annealed 54 hr. at 147 600 0. prior to cold drawing to .081. do 107 306 354 367 290 325 .do 365'. Nil .162 rod annealed 1 hr. at 3:5

850 C. and quenched prior to cold drawing to .081".

TABLE II Commercial I oxygen-bean so I wug f i i Thermal treatment temp containing per cent Cd .034 94s" rod annealed 1 hr. at 500 C. and 268 drawn to .081 for test. ...do 261 .061.. do.. 263 .034. tie" rod annealed at 850 C. for 1 hr., 845

quenched, and drawn to .081. .044 do 345 .061 d0 347 .034 fie rod annealed at 850 0., 332

quenched and drawn to .162", anneal at 400 C. for 1 hr. .034 Same treatment with l hr. at 400 C. 332

The effect of small additions of cadmium gon the softening temperature of copper is v ry marked as shown in Tables I- and Where oxygen-free copper is used, the softening temperature rapidly increases with small increments of cadmium. Copper containing oxygen, however, does not show as large a rise in softening temperature with similar cadmium additions. This difference is due to the partial precipitation of cadmium as an oxide. We have found-that the maximum effect of the cadmium is not obtained unless it is present in solid solution. By

annealing the oxygen-bearing alloys at 850 C.-

for one hour, or 1000 C. for hour and quenching before drawing to .081", it was observed that the softening temperature values obtained agreed very closely with those determined for the oxygen-free alloys.

This rise in softening temperature is accredited to an increase in the amount of cadmium present in solidsolution and the only source of such cadmium is the cadmium oxide precipitated in the copper. The reaction which occurs is indicated as follows:

As the temperature is lowered, the reaction starts proceeding from left to right and cadmium oxide is precipitated. Since reactions occurring at lower temperatures always require considerably more time, due to a decrease in difl'usion rates, this would indicate that the cadmium was only partially precipitated as cadmium oxide during the /2 hour anneal at 600 C. as shown by the test results in Table I.

The importance of high temperature quenching in order to convert the cadmium to its ef-' fective solid solution form is noted in the differences obtained'from the 500 C. and 850 C. heat treatments shown in Table II. Annealing at the lower temperature permits appreciable amounts of cadmium oxide to be formed and precipitated rendering the cadmium ineffective for raising the softening temperature.

In Table III the effects of varying the quench ing temperature are tabulated for comparison:

TABLE III Efiect of varying the quenching temperature (lomtmwgigal cas Softening bar cop r Thermal treatment eonteini n g temp per cent Cd 1 hr. at 700 C. and quenched 300 1 hr. at 750 C. and quenched 320 1 hr. at 800 C. and quenched 330 1 hr. at 700 C. and quenched 298 1 hr. at 750 C. and. 313 1 hr. at 800 C. and 323 1 hr. at 700 C. and 312 1 hr. at 750 C. and 330 1 hr. at 800 C. and 334 1 hr. at 700 C. and 307 1 hr. 750 C. and 333 1 hr. at 800 C. and 340 1 hr. at 700 C and 312 1 hr. at 750 C. and 328 1 hr. at 800 C. and 341.

As observed in TableIiI, there is a definite tendency for the lower quenching temperature to be less effective in holding the cadmium in solid solution. It is essential, in order to obtain the highest increase in softening temperatures to have all the cadmium present'in solid solution. Broadly, our process contemplates heat treating coppers containing one or more extraneous metals of the type which form metal oxides in oxygenbearing copper and are reducible in situ by heating at temperatures below the melting point of copper, such as cadmium, antimony, lead and the like, to raise the temperature at which the copper can be safely heated without encountering undue softening. This treatment consists in heating the copper at temperatures ranging from 700 C to 1000 C. until a substantial amount of the extraneous metal oxide has been reduced to the metallic form and is in solid solution, and then cooling the copper sufliciently rapidly from this temperature to maintain it in solid solution.

In this connection, it is to be pointed out that the length of time allowed at the annealing temperature has a bearing on the results obtained. For example, annealing fifteen minutes between 850 C. and 1000 C. may be used with good results, whereas by employing a lower temperature, i. e. 700 C. or lower, an hour or more is required to insure retention of the desired quantity of cadmium in solid solution.

When cadmium is added to pure copper, the loss of conductivity in either the quenched or unquenched condition is so small as to be negligible. Wirebar coppers alter the results somewhat because of the presence of additional extraneous substances which are themselves responsible for a certain loss of conductivity. The comparative restivity of commercial wirebar coppers with and without cadmium as determined after annealing for 20 minutes at 500 C. according to A. S. T. M. specification are given in the following table:

From the test results of Table IV, it is apparent that cadmium bearing coppers can be made which are well above the A. S. T. M. requirements for high conductivity copper. As observed from the test data,.no measurable lowering of the conductivity results when the cadmium content is below 0.1%. The preferred cadmium content is from 0.01% to0.1%.

The improved results attained by introducing small amounts of cadmium in coppers areparticularly applicable to the manufacture of copper known to the trade as tough pitch copper and conventionally utilized for the production of cast copper ingots, wirebars, cakes, etc. This a product contains usually on the order oi 0.025%

to 0.070% oxygen. y

We have found by'tests that an alloy containing .02% cadmium can be advantageously substituted for silver bearing copper containing 10 ounces of silver perton. This copper-cadmium alloy, when annealed and quenched from 750to 850 C., is superior to the silver bearing product in its resistance to softening. Process annealing at 400 C. can be used as necessary in the mill schedule, and the conductivity resulting from this treatment surpasses that required for high conductivity coppers. It has also been observed by tests that cadmium-bearing wirebars can be cast without difiiculty with respect to set and appearancer What is claimed is:

1. The process of treating copper containing oxygen in amount providing a tough-pitch copper to increase the softening temperature thereof, which comprises including cadmium therein in efiective amounts but not exceeding substantially 0.1%, heating the cadmium-bearing copper 'to within a temperature range of approximately C. to 1000 C., maintaining the said copper oxygen to increase its softening temperature,

which comprises alloying with said copper from .01% to 0.1% of cadmium, heating the resulting copper to a temperature not less than substantially 700 C., maintaining the said temperature untiI substantially all of the cadmium has been converted into solid solution in the copper, and

cooling the resulting heat-treated copp r from said temperature sufllclently rapidly to maintain the cadmium in solid solution.

3. The process of treating commercial tough pitch copper containing from 0.025% to 0.07% oxygen and small but effective amounts of extraneous metal oxide which is reducible in situ at temperatures below the melting point of said copper to increase its softening temperature, which comprises heat-treating said copper at temperatures ranging from substantially 700 C. to about 1000' C. until said extraneous metal oxide which includes cadmium oxide is converted substantially completely into solid solution in the copper, and quenching the copper from such temperatures to maintain thesaid extraneous matter in solid solution in the copper.

4. A new metallurgical product composed of commercial tough-pitch copper containing small but eflective amounts of cadmium but not more than approximately 0.1% thereof, the cadmiumbeing present substantially entirely in solid solution in the copper, said copper containing from 0.025% to 0.07% oxygen.

5. A new metallurgical product composed of commercial oxygen-bearing copper containing .from 0.025% to 0.07 oxygen and from approximately 0.01% to 0.10% cadmium which is present at least substantially entirely in solid solution in said copper.

ALBERTA. SMITH, JR. JOHN S; SMART, JR. 

